2.1 Allergic diseases
The overall patterns of immune responses to allergens indicate that T lymphocytes are central in the generation of allergic diseases (Romagnani, 1992, Immunol. Today 13:379-380; O'Hehir et al., 1991, Ann. Review Immunol. 9:67-95). Following allergen processing and presentation by antigen presenting cells (APA) they function as helper T cells in cooperating with B cells to produce antibodies such as immunoglobulin IgE. Receptors on mast cells and basophils bind allergen specific IgE. Subsequent exposure to allergen results in the release of inflammatory molecules which cause allergic symptoms.
Allergen sensitized T lymphocytes also function directly in initiating allergic responses. This includes the release of cytokines such as interleukin 5, so promoting eosinophil mediated allergic responses in asthma and related diseases. They also initiate local inflammatory responses such as those caused by exposure to contact sensitizing allergens. Environmental sources of contact sensitizing allergens include naturally occurring compounds such as alkylcatechols present in poison oak and ivy as well as pollutants and industrial chemicals.
2.1.1 IgE mediated diseases: Asthma, allergic rhinitis and atopic dermatitis
Classic immediate type allergic reactions are mediated by the IgE subclass of antibodies, and are directed against a variety of environmental allergens. The most common of these world-wide are dust mite, pollen from grasses, trees, broad-leaf weeds, mold spores, and animal dander. After initial sensitization, exposure results in an immediate reaction caused by the release of a range of inflammatory molecules including histamine and prostaglandins from mast cells. A second wave of cellular activation occurs several hours later, manifest in the skin as vasculitis with leukocyte infiltration, and in the lungs by eosinophil and lymphocyte infiltration. The second phase reaction is thought to be responsible for promoting the chronic changes and hypersensitivity which often accompanies the allergic diseases.
2.1.2 Dust mite allergens
Over the last several years, evidence of the strong involvement of dust mite antigen (DMA) allergy in allergic rhinitis and especially asthma has strengthened. In a recent study (Gergen and Terkeltaub, 1992, J. All. Clin. Immunol. 90:579-588) it was shown that allergic rhinitis without asthma was associated with skin test reactivity to only three allergens, dust mite (house dust), ragweed, and rye grass. Asthma alone was only associated with reactivity to house dust and alternaria. Additionally many reports have documented the close physical association between dust mites in the home and asthma (Sporik et al., 1992, Clin. Exp. All. 22:897-906; Platts-Mills, 1992, J. All Clin. Immunol. 89:1046-1060).
There are two major species of dust mite, Dermatophagoides pteronyssinus (Der p) and Dermatophagoides farinae (Der f). A range of 30 proteins have been identified in allergenic preparations derived from each species; but only two, group 1, Mw 25,000, and group 11, Mw 14,000 are recognized as the major mite allergens. Within each group, proteins are highly cross reactive between the two species. These two proteins account for 60-70% of the reactivity of the IgE in allergic individuals.
2.1.3 Rye grass allergens
Grass and weed allergens are important causes of allergic rhinitis and asthma. Most of the grasses are cultivated agriculturally or ornamentally and are prevalent in residential areas. Only about a dozen of more than 5000 species of grasses are important allergens since many of the grasses do not produce abundant pollen. Important allergenic grasses include Rye, Timothy, Kentucky blue and June. Many of the allergens of different species cross react antigenically, and show amino acid sequence homology and may share T and B lymphocyte epitopes.
Rye grass (Lolium perenne) which is widely distributed in temperate zones induces allergic reactions in pollen sensitive individuals. Five groups of allergens (Lol p I, II, III, IV, and V), some in multiple forms, have been identified (Marsh et al., 1987; Thomas, 1991; Mathiesen et al., 1991). Lol p I is a major allergic species. Elevated levels of Lol p I specific IgE have been found in sera of up to 95% of grass pollen-allergic subjects (Freidhoff et al., 1986, J. All. Clin. Immunol. 78:1190-1201). Lol p I is an acidic glycoprotein of molecular weight 34 kDa, located in the cytosol of the pollen. Certain IgE immunodominant epitopes have been identified. For example, a C-terminal peptide (aa 213-240) obtained by tryptic digestion of Lol p I has been found to contain an IgE binding site. Synthetic peptides from the Lol p I C terminus (aa 216-240) also contain IgE and IgG binding sites (Van Ree et al., 1992).
2.1.4 TC mediated allergic skin disease: Poison oak and ivy dermatitis
Poison oak and ivy dermatitis is one example of a direct T cell mediated allergic reaction. This reaction occurs against the hapten urushiol when it contacts the skin, bronchial epithelium or the like. The hapten is a mixture of 3-n-alkylcatechols found in the oil of the plant, which conjugates to body proteins. The conjugate stimulates T cells to directly attack the skin or other cells to which the hapten is bound.
T cell etiology of the disease has been confirmed by studies in animal models of poison oak dermatitis which include the guinea pig and mouse. Transfer studies in animals have demonstrated the allergic reaction can be transferred from sensitized to naive animals by T lymphocytes. Human and animal studies have identified the specificity of the T cell reaction against the allergen. Both the specificity and antigenicity of the compounds reside primarily in the common catechol structure. The immunologic reactions seen with urushiol are typical of those caused by various identified contact sensitizers, such as chlorinated hydrocarbons and epoxy resins.
In humans, oral hyposensitization is quite effective although cumbersome since it requires 3-6 months of daily administration of the oil. Although antibodies against urushiol have never been directly demonstrated, the antibody fraction of sera from desensitized patients will transfer tolerance to mice, suggesting a role for regulatory antibodies.
2.2 IgG mediated response to protein antigens: RTA
There are other cases in which undesirable immune responses occur to protein antigens, such as ricin A chain (RTA) a protein which, when coupled to a targeting agent such as a lymphokine or monoclonal antibody such as Mab 791T/36 (immunotoxin), directed against colorectal cancer, can kill the targeted cell. RTA is a very potent immunogen, and a single injection of immunotoxin in animals or humans produces a vigorous IgG immune response against all parts of the molecule.
2.3 Immunology of allergic diseases
T lymphocytes, predominantly the CD4+ subset (Th2), play a central role in the initiation and maintenance of aero-allergen-mediated immune responses (Peltz, 1991; O'Hehir et al., 1991; Romagnani, 1992). Following exposure of susceptible subjects, T lymphocytes collaborate with B lymphocytes in producing cytokines. These interact with T and B cells, as well as other cells of the immune network such as macrophages. The lymphokines stimulate the enhanced production of helper T cells and B cells. Helper T cells further differentiate into two distinct subsets (Th1 and Th2) with characteristic lymphokine profiles. Th2 cells produce interleukin 4 (IL4), which is one signal required for IgE antibody synthesis. A second signal for IgE synthesis is produced following cognate T-B cell interactions involving recognition of T cell receptor/CD3 complex and major histocompatibility complex class II molecules (Romagnoni, 1990; Geha, 1992).
In addition to their role in IgE production, Th2 lymphocytes produce other lymphokines, particularly interleukin 5 (IL5), which are required for eosinophil recruitment and for initiating eosinophil mediated toxic responses (Clutterbuck et al., 1988, Blood 73:1504; Lopez et al., 1988, J. Exp. Med. 167:219). Il5 also-promotes most cell degranulation with release of cytokines, basic granule proteins, and platelet-activating factor. The T lymphocyte is critical for both immediate and late phase reactions, but is more directly involved in the late phase. This is demonstrated by the fact that steroids are active in preventing the late phase, but not the immediate phase reaction.
This overall pattern of immune responses to allergens implicates T cells as pivotal in the generation of immunoglobulin E and eosinophilia, two major components of an allergic response to aero-allergens (O'Hehir et al., 1991, Ann. Rev. Immunol. 9:67-95). Control of T cell responses to allergens through intervention at any of several points in the immune pathway therefore constitutes a major approach to treating allergic diseases. This is illustrated by the results from treatment of severe chronic asthmatics with cyclosporin A. Cyclosporin A is a potent immunosuppressive agent which acts principally on T lymphocytes. Patients treated with cyclosporin A showed a clinically significant improvement in their asthma symptoms.
2.4 Immunotherapy with allergens
Immunotherapy of allergic diseases including asthma, allergic rhinitis, as well as urushiol dermatitis has been undertaken for many years using hyposensitization with crude allergen extracts. This form of immunotherapy is effective in many patients and can provide lasting benefit even after immunotherapy has been discontinued (Busse, 1988, J. All. Clin. Immunol. 82:890-900). Immunotherapy with grass extracts has proven useful in treating allergic rhinitis and asthma (Creticos and Norman, 1987, JAMA 258:2874-2880; Creticos, 1989, J. All. Clin. Immunol. 83:554-562; Reid et al., 1986, J. All. Clin. Immunol. 78:590-600). In like fashion, patients with severe asthma achieved significant improvement following treatment with DMA-antibody complex (Machiels et al., 1990, J. Clin. Invest. 85:1024-1035), also clinical improvement and decreased bronchial sensitivity on bronchial provocation in children immunized for three years with a house dust extract (Creticos and Norman, 1987, JAMA 258:2874-2880, Creticos, 1990, J. All. Clin. Immunol. 83:554-562; Bousquet et al., 1985, J. All. Clin. Immunol. 76:734-744), and in patients with allergic rhinitis desensitized with either purified AgE from ragweed, or with the whole extract (Norman et al., 1968, J. All. 42:93-108). Similarly, with poison oak/ivy urushiol, oral hyposensitization is quite effective although cumbersome since it requires 3-6 months of daily administration of the oil. Indirect evidence for a regulatory role for anti-idiotypic antibodies and/or regulatory T cells is provided by several experimental and clinical studies. Subjects hyposensitized with urushiol generate immunoglobulins which can transfer tolerance to mice (Stampf et al., 1990, J. Invest Derm.). However, there are numerous difficulties with this form of treatment. Allergen extracts are crude, so that treatment schedules are not able to be standardized. Also, prolonged courses of treatment result in low patient compliance. Since the precise immune mechanism activated by hyposensitization with allergic extracts is not known, the cause of therapeutic failures usually cannot be established.
Several mechanisms may be postulated whereby immunoregulation of T cell dependent allergic responses can be achieved. One mechanism is to interfere with antigen processing and presentation to the T cell. Another is to tolerize the T cell, either directly or by production of T suppressor cells. This process will block either cognate T-B cell interactions or production of cytokines.
Indirect evidence for a regulatory role for anti-idiotypic antibodies and/or regulatory T cells is provided by several experimental and clinical studies. Subjects hyposensitized with urushiol generate immunoglobulins which can transfer tolerance to mice (Byers et al., 1990). In immediate type hypersensitivity, a general finding during hyposensitization is an initial increase in IgE antibodies, followed by a decrease. Concomitantly, there is an increase in the allergen specific IgG (Creticos, PS, JAMA 268:2834-2839). When the specificity of the response is investigated carefully, it is found that auto-anti-idiotypic antibodies develop (Gurka et al., 1988, Ann. Allergy 61:239-243). This has been shown to be the case in rye grass hyposensitization as well. Clinically, anti-idiotypic antibodies are elevated in human subjects following hyposensitization with rye grass extracts concomitant with reduction in allergic rhinitis (Hebert et al., 1990, Clin. Exp. Immunol. 80:413-419).
2.4.1 Immunotherapy with anti-idiotypic antibodies (Ab 2)
While transfer of anti-idiotypic antibodies results in suppression of both T and B lymphocyte initiated response, immunotherapy with monoclonal or polyclonal anti-idiotypic antibodies has not been successfully developed for allergic diseases, since the responses in experimental systems have been contradictory. In one example, treatment of mice with polyclonal anti-idiotypic antibodies generated against a mouse monoclonal antibody reacting with a tobacco mosaic virus protein polypeptide suppressed the ability of mice to generate antibody responses to the decapeptide (Norton and Benjamini, 1987, Cell Immunol. 109:418-419). In another example, polyclonal anti-idiotypic antibodies produced by immunization of guinea pigs with polyclonal anti-benzylpenicilloyl antibody were shown to down regulate the anti-benzyl penicillin IgE antibody responses when given either pre or post sensitization (Wetterwald et al., 1986, Mol. Immunol. 23:347-356). Also, polyclonal anti-idiotypic antibodies to a monoclonal antibody which reacted with a decapeptide of tobacco mosaic virus suppressed the induction of antibodies (the class is unknown) to this decapeptide in mice, when administered prior to sensitization (Norton and Benjamini, 1987). Clinically, transfer of serum from bee keepers who had tolerated multiple bee stings into individuals who previously developed anaphylaxis to desensitization allowed the receipts to undergo successful desensitization (Bousquet, 1987, J. All. Clin. Immunol. 79:947-954), and it is shown that the transferred whole sera contained anti-idiotypic antibodies against bee venom (Boutin et al., 1993, J. All. Clin. Immunol. in press). This has been explained by transfer of Ab2s which down regulated the response. Recently it has been confirmed that in one patient successfully treated in this way, the transferred whole sera did contain anti-idiotypic antibodies against bee venom. A decrease in skin test reactivity also was observed after transfer (Hebert, 1991).
In general, however, treatment with monoclonal Ab2 antibodies has been undertaken to up-regulate responses to specific antigens. These include cancer and viral antigens (Fung et al., 1990, J. Immunol. 145:2199-2206; Raychaudhuri et al., 1990, J. Immunol. 145:760-767, reviewed in Kohler et al., 1989, Clin. Immunol. and Immunopath. 52:104-116). Several of these are in clinical trials, for example against cancer antigens (Robins et al., 1991, Can. Res. 51:5425-5429). Consistent with these findings, vaccination with a monoclonal Ab2 directed against Lol p I allergen produced up-regulation of the IgE and IgG anti-Lol p I antibodies (Boutin et al., 1991, J. All. Clin. Immunol. 87 (abstract) page 197). In all cases the up-regulation has been explained by the induction of AB3 antibodies.
2.4.2 Immunotherapy with Ab1 antibodies
Regulation of the immune response by Ab1s has been investigated. Monoclonal antibodies against bovine serum albumin, given to virgin mice in the absence of adjuvant, could down-regulate the IgG response to later immunization with that antigen (Eddy et al., 1987, J. Immunol. 138:1693-1698). Polyclonal antibodies against sheep red cells administered pre-sensitization could also nonspecifically down-regulate the IgG response to that antigen (Heyman and Wigzell, 1984, J. Immunol. 132:1136-1143). The authors suggested that this effect was due to nonspecific antigen blockade by the Fc portion of the molecule. However, these studies did not demonstrate modulation of an established immune response.
Saint-Remy et al. have shown in several publications (Macheils et al., 1990, J. Clin Invest. 85:1024-1035; Saint-Remy et al., 1988, Eur. J. Immunol. 18:1009-1014) that autologous antigen-antibody complexes, made by adding antigen to sera from patients who were refractory to hyposensitization, could down-regulate the IgE response to a variety of allergic diseases including asthma and atopic dermatitis. After administration of antigen-antibody complexes, the level of anti-idiotypic antibodies was increased; this was associated with clinical improvement.
The mechanism of action in these studies has not been well investigated. Multiple mechanisms are probable, including nonspecific blockage of antigen processing via the Fc receptor, and antigen specific blockage of antigen processing (Heyman et al., 1990, Immunol. Today 11:310-313; Wiersma et al., 1989, Scan J. Immunol. 29:439-448; Sinclair et al., 1987, Immunol. Today, 8:7679). However it would appear that polyclonal antisera can down-regulate the IgG response. The use of monoclonal antibodies to down-regulate the IgE response is considerably more complex, in part because of the difficulty in selecting the specificity of the monoclonal antibody.